The recent emphasis on cleaner energy technologies has focused new attention on hydrogen as an alternative fuel, especially for use in proton exchange membrane (PEM) fuel cells. Currently, most hydrogen worldwide is produced from hydrocarbon sources such as natural gas, oil, and coal, with only 4% originating from water electrolysis. Hydrogen produced from these sources will contain residual hydrocarbons, carbon monoxide and sulfur compounds, all of which can rapidly poison the PEM catalyst in concentrations as low as 10 ppm. An economic way to remove contaminants from hydrogen is therefore desirable. Membrane separation technologies have the potential to reduce operating costs, minimize unit operations, and lower energy consumption. Dense membranes made of palladium, nickel, platinum, and the metallic elements in groups III-V of the periodic table are able to transport hydrogen in a dissociated form, and are thus capable of theoretically infinite selectivity. Palladium membranes are of particular note for their high permeability, tolerance to hydrocarbon-containing streams, and their ability to self-catalyze the H2 dissociation reaction.
The commercial application of palladium membranes has been limited by several factors. Pure palladium undergoes an embrittling hydride phase transition when exposed to hydrogen at temperatures below 300° C. Furthermore, it is subject to deactivation by carbon compounds at temperatures above 450° C. and irreversible poisoning by sulfur compounds. Additionally, the cost of commercial palladium foils (particularly those greater than 25 microns in thickness) is high. Because a successful membrane requires a lifetime on the order of several years under operating conditions, these concerns must be addressed.
In order to reduce poisoning and embrittlement issues, palladium can be alloyed with a variety of other metallic elements. Alloy membranes have been made with such materials as Ag, Au, Cu, Fe, Ni, Pt, and Y. As shown in FIG. 1, the pioneering work done by McKinley (U.S. Pat. No. 3,350,845, 1967; and U.S. Pat. No. 3,439,474, 1969) demonstrated that certain copper and gold alloys actually have higher H2 permeability than pure palladium, are unaffected by thermal cycling, and have improved resistance to poisoning by hydrogen sulfide. In the case of PdCu, the membrane has a sharp peak in permeability at the 40 wt % Cu composition, requiring precisely controlled fabrication in order to maximize hydrogen throughput.
Palladium alloys are traditionally produced by cold working, which allows for highly precise composition control but requires costly equipment in order to produce foils of less than 25 microns thickness. Therefore a great deal of research has been dedicated to alternative methods of production such as electroless plating, electroplating, physical vapor deposition (PVD), and chemical vapor deposition (CVD). Of these, electroless plating is the most heavily researched since it requires minimal equipment, no exotic precursors, and can be performed on any appropriately activated surface. Alloys can be either coplated or sequentially plated and then alloyed by annealing.
In one typical fabrication strategy, Pd-alloy membranes are plated on porous support media in order to combine the high flux of a thin film with the mechanical strength of the support. The most common support materials are porous ceramic or sintered porous metal, the latter of which typically uses an oxide layer between the support and the membrane to prevent intermetallic diffusion. Membranes with a thickness less than one micron can be produced upon these porous supports. But in some applications it is desirable to retain the intrinsic material properties of the palladium support, such as composition, crystal structure, and gas sorption, without the influences of the support.
Therefore, there exists a need for free-standing palladium membranes capable of selectively transporting hydrogen gas in the absence of support media while withstanding temperature and pressure cycles encountered in typical applications in which hydrogen purification membranes are used, and well-controlled methods of efficiently and economically producing these membranes.